A photocatalytic reaction is a reaction in which excited electrons and holes (positive holes: holes) out of which the electrons have gone are generated upon irradiation of a solid compound having a photocatalytic activity with an ultraviolet ray, whereby a reactant is oxidized or reduced because the excited electrons each have a reducing action, and the holes each have a strong oxidizing action. Titanium dioxide is known as a typical solid compound having a photocatalytic activity. Titanium dioxide can exhibit a strong oxidizing action upon absorption of an ultraviolet ray and is thereby applied in a wide variety of applications such as air purification, water purification, pollution control, deodorization, antimicrobe, prevention of hospital infection, and demisting.
Rutile and anatase crystal forms are known as major crystal forms of titanium dioxide. These crystalline titanium dioxides display higher chemical stability and larger refractive indices than those of amorphous titanium dioxide (amorphous). In addition, the crystalline titanium dioxides are easily controllable on shape, size, and degree of crystallinity, of crystals.
It has been also known that titanium dioxide particles each having a high degree of crystallinity can exhibit a superior photocatalytic activity to that of titanium dioxide powders each having a low degree of crystallinity; and that, with an increasing crystal size, the titanium dioxide particles exhibit more satisfactory photocatalytic activity.
Patent Literature (PTL) 1 describes a method for preparing a titanium dioxide crystal having a novel exposed crystal face being developed by subjecting titanium dioxide to a treatment with a basic hydrogen peroxide solution, a treatment with sulfuric acid, or a treatment with hydrofluoric acid; and mentions that a photocatalyst including the prepared titanium dioxide having a newly exposed, novel exposed crystal face has a high oxidative catalytic capability. The literature discloses, as examples of the titanium dioxide having a novel exposed crystal face, (1) a titanium dioxide crystal obtained from rutile titanium dioxide and having a newly exposed (novel exposed) (121) face; (2) a titanium dioxide crystal obtained from rutile titanium dioxide and having newly exposed (001), (121), (021), and (010) faces; (3) a titanium dioxide crystal obtained from rutile titanium dioxide and having a newly exposed (021) face; (4) a titanium dioxide crystal obtained from anatase titanium dioxide and having a newly exposed (120) face; (5) a titanium dioxide crystal obtained from anatase titanium dioxide and having a newly exposed (122) face; and (6) a titanium dioxide crystal obtained from anatase titanium dioxide and having a newly exposed (112) face.
However, such titanium dioxide catalysts having conventional crystal forms do not sufficiently exhibit catalytic actions in some applications, and demands have been made to provide a titanium dioxide photocatalyst having a higher catalytic activity.